Signal processing method for a towed linear antenna

ABSTRACT

The disclosure relates to a signal processing method for a towed linear antenna, notably to resolve right-left ambiguity, said antenna comprising a set of hydrophone multiplets each with n≧3 hydrophones spread over a straight section of the longitudinal axis of said antenna, wherein the roll angle of each multiplet relative to the vertical is measured, the signals of said hydrophones and said roll angles are used to synthesize p≧3 linear sub-antennas, then M azimuth channels are formed with each sub-antenna and 2 adaptive right-left channels are formed from said p sub-antennas for each direction corresponding to each azimuth channel. The invention resolves right-left ambiguity with optimal performance in detection and for long antennas.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method of processing signalsfrom a towed linear antenna. It notably resolves right-left ambiguity onsuch an antenna when it is activated, in other words when it isreceiving echoes produced from emissions of an active sonar.

[0002] There are known techniques for resolving this right-leftambiguity, described in particular in the French patent no.8911749 filedby the company THOMSON-CSF on 8 Sep. 1989 and published on 15 Mar. 1991under the number 2 651950 and delivered on 17 Apr. 1992, which consistin replacing each omnidirectional hydrophone by at least 3 hydrophonesmounted in a plane perpendicular to the axis of the linear antenna andspread around a circle, inside the antenna and centered on its axis. Inthis manner, a volumic antenna is constituted enabling construction ofantenna responses with right-left rejection capabilities. In particular,the processing of the signals enables a “zero” to be created in theambiguous direction relative to the setpoint direction of the channel.

[0003] To remove the ambiguity satisfactorily using this technique, itis necessary that the “triplets” of hydrophones (or more generally the“n-multiplets”) be aligned so as to constitute a set of 3 (n) linearsub-antennas. This linearity can be assured by means of rigid linksbetween the triplets, but this has the disadvantage of working well onlyfor antennas that are not too long. However it is increasingly common toreduce the working frequency in order to increase the range, but thisreduction generally implies increasing the length of the antennasproportionally, in which case it becomes increasingly difficult tocontrol the linearity of the sub-antennas.

[0004] Moreover, since the antenna diameter is small compared with thewavelength, the creation of a zero leads to signal losses that becomelarger at lower frequencies, in particular when the dominant noise isdecorrelated between hydrophones of the same triplet, which represents asecond very serious disadvantage.

SUMMARY OF THE INVENTION

[0005] The processing according to the invention overcomes thesedrawbacks, notably by:

[0006] eliminating the effects of rotation of the triplets (orn-multiplets) relative to each other,

[0007] ensuring a gain between the formation of an antenna channel andthe formation of a right-left channel, regardless of the dominant noiseconditions.

[0008] For this purpose, the object of the invention is a signalprocessing method for a towed linear antenna, notably to resolveright-left ambiguity, said antenna comprising a set of hydrophonemultiplets each with n≧3 hydrophones. spread over a straight section ofthe longitudinal axis of said antenna, wherein the roll angle of eachmultiplet relative to the vertical is measured, the signals of saidhydrophones and said roll angles are used to synthesize p≧3 linearsub-antennas, then M azimuth channels are formed with each sub-antennaand 2 adaptive right-left channels are formed from said p sub-antennasfor each direction corresponding to each azimuth channel.

[0009] According to another characteristic of the invention, a coherenttreatment is used to form said M azimuth channels.

[0010] According to another characteristic of the invention, to formsaid right-left channels the inverse interspectral matrix {circumflexover (Γ)}⁻¹ is estimated on N+1 time samples and A+1 Doppler channels,then two dephasing vectors d_(r) and d_(l) are determined associatedwith the right and left channels for each azimuth channel, and thechannels V_(right) and V_(left) are determined from these vectors andthis inverse matrix.

[0011] According to another characteristic of the invention,inclinometers are used to determine the roll angles.

[0012] According to another characteristic of the invention, n=p=3.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention will be better understood and its otherparticularities and advantages will become clear on reading the detaileddescription below of an embodiment, which is non-limitative and takenonly as an example, with reference to the attached drawings of which:

[0014]FIG. 1 is a block-diagram illustrating the process according tothe invention;

[0015]FIG. 2 is a graph defining the roll angle used in thecalculations;

[0016]FIG. 3 is a block-diagram developing the steps 102 and 103 of FIG.1;

[0017]FIG. 4 is a block-diagram developing step 104 of FIG. 1;

[0018]FIG. 5 is a graph defining other angles used in the calculations.

MORE DETAILED DESCRIPTION

[0019] The processing according to the invention includes 4 modules ofsteps, as shown in FIG. 1.

[0020] A first module 101 receives the hydrophonic signals from all thehydrophone multiplets of the antenna sampled in a first preliminarymodule 100 and the roll angles R measured in a second preliminary module110 at each multiplet (or only some of them) by means of a controlledsensor, for example an inclinometer, as defined in FIG. 2.

[0021] The treatment carried out in this module 101 consists in formingusing the n hydrophones of each multiplet and the indication of theirroll angle R, p virtual hydrophones having a fixed angular positionindependent of the multiplet considered, so as to reconstitute p linearantennas to obtain dynamic roll stabilization of the whole antenna.

[0022] To do this, the hydrophone signals are interpolated spatiallyusing a known technique. In a preferred embodiment, n=3 and p=3, whichcorresponds to the well-known triple antenna. The steps 102 and 103 inwhich the azimuth channels on each sub-antenna are formed and eachchannel signal is treated coherently are implemented in the frequencydomain in 4 modules of steps, as shown in FIG. 3. These calculationsteps are known from the prior art.

[0023] In the case where the emitted code is sensitive to the Doppler,the coherent treatment is a multicopy treatment known from the priorart, described for example in the French patent published under thenumber 2 6787 226.

[0024] Described succinctly, the hydrophonic signals stabilizeddynamically in the module 101 are converted from time space intofrequency space by a fast Fourier transform (FFT) 201. Next, in a knownmanner the azimuth channels are formed in a module 202, then the productof the conjugated spectra of the Dopplerized emission signal and thespectrum of each channel is computed in a module 203. An inverse FFT isperformed in a module 204 to obtain the channel signals treatedcoherently, corresponding to those delivered by the module 103.

[0025] In a module 104, an adaptive channel is formed on each azimuthchannel obtained at the output of stage 103.

[0026] The azimuth channels are then distributed according to the cosineof the azimuth varying from −1 to +1: if u designates the cosine of theazimuth, the pointing directions obey the relation:

u _(m)=(2m+1)/M _(v)

[0027] where ${{- \frac{M_{v}}{2}} \leq m \leq {\frac{M_{v}}{2} - 1}},$

[0028] and M_(v) is the number of azimuth channels.

[0029] The output signal of channel m of the linear antenna I (1≦I≦3) attime t will be denoted V_(l)(m,t,α), and {right arrow over (V)}(m,t,α)will designate the column vector constituted by the 3 components V₁, V₂,V₃ for the azimuth m at time t with a “Doppler” of α.

[0030] The “Doppler” α is defined as the Doppler shift 2{right arrowover (V)}/C, where {right arrow over (V)} is the resultant vector of theradial velocities of the emitter and the receiver and C is the speed ofsound propagation in water. Its range of variation is determined by 2values of α corresponding to extreme speeds. It is known that the widthof a Doppler channel is given by:

Δα=1/f ₀ T

[0031] where f₀ and T are respectively the central frequency and theduration of the emission code. A+1 adjacent Doppler channels are thusdefined between the 2 extreme values of α.

[0032] The module 104 comprises 3 modules of sub-stages represented inFIG. 4:

[0033] estimation of the interspectral matrix (401);

[0034] inversion of the interspectral matrix (402);

[0035] formation of the 2 right-left channels (403).

[0036] In the module 401, the interspectral matrix {circumflex over(Γ)}(m,t,α) is estimated by averaging over N+1 time samples and over A+1Doppler channels, applying the formula:${\hat{\Gamma}\left( {m,t,\alpha} \right)} = {\sum\limits_{\alpha^{\prime} = {{- A}/2}}^{A/2}{\sum\limits_{t^{\prime} = {{- N}/2}}^{N/2}{{\overset{\rightarrow}{V}\left( {m,{t + t^{\prime}},{\alpha + \alpha^{\prime}}} \right)} \cdot \left\lbrack {\overset{\rightarrow}{V}\left( {m,{t + t^{\prime}},{\alpha + \alpha^{\prime}}} \right)} \right\rbrack^{+}}}}$

[0037] the notation “+” designating the conjugated transposed matrix.Advantageously, the time coverage is 50%.

[0038] The matrix {circumflex over (V)}(m,t,α) having a small dimension(p), the estimation requires about 1.5 p to 2 p independent observationsand is therefore performed over a very short time period, enabling thetreatment to adapt to environmental variations (noise, reverberation)and also to take into account the echo and reject it in the channels ofdirection opposite to its arrival direction.

[0039] In the module 402, the inverse matrix {circumflex over(Γ)}⁻¹(m,t,α) is computed using a known direct method.

[0040] Finally, in the module 403 the left and right channels V_(left)and V_(right) are formed, in the manner explained below in the case of atriplet.

[0041]FIG. 5 shows the notations used in this case of a triplet: “a” isthe radius of the triplet; θ and φ are the angles corresponding to thepointing direction of a channel when it is formed.

[0042] To do this, in the module 403, the 2 channels V_(left) andV_(right) are formed by first calculating the 2 dephasing vectors d_(r)and d_(l) associated with the right and left channels for each azimuthchannel.

[0043] For pointing in the horizontal plane, in other words φ=0, these 2vectors are given by the following equations:${{\underset{\_}{d}}_{l}(m)} = \begin{bmatrix}{\exp \left\lbrack {{2}\quad \pi \quad a\frac{f_{0}}{c}\sin \quad \theta_{m}\quad {\cos \left( {R - \frac{\pi}{2}} \right)}} \right\rbrack} \\{\exp \left\lbrack {{2}\quad \pi \quad a\frac{f_{o}}{c}\sin \quad \theta_{m}\quad {\cos \left( {\frac{2\pi}{3} + R - \frac{\pi}{2}} \right)}} \right\rbrack} \\{\exp \left\lbrack {{2}\quad \pi \quad a\frac{f_{o}}{c}\sin \quad \theta_{m}\quad {\cos \left( {\frac{4\pi}{3} + R - \frac{\pi}{2}} \right)}} \right\rbrack}\end{bmatrix}$ ${{\underset{\_}{d}}_{r}(m)} = \begin{bmatrix}{\exp \left\lbrack {{2}\quad \pi \quad a\frac{f_{0}}{c}\sin \quad \theta_{m}\quad {\cos \left( {R + \frac{\pi}{2}} \right)}} \right\rbrack} \\{\exp \left\lbrack {{2}\quad \pi \quad a\frac{f_{o}}{c}\sin \quad \theta_{m}\quad {\cos \left( {\frac{2\pi}{3} + R + \frac{\pi}{2}} \right)}} \right\rbrack} \\{\exp \left\lbrack {{2}\quad \pi \quad a\frac{f_{o}}{c}\sin \quad \theta_{m}\quad {\cos \left( {\frac{4\pi}{3} + R + \frac{\pi}{2}} \right)}} \right\rbrack}\end{bmatrix}$

[0044] where f_(o) is the transmit frequency.

[0045] The channels V_(left) and V_(right) are then formed by applyingthe following equations: $\begin{matrix}{{Left}\quad {channel}\text{:}} & {{V_{left}\left( {m,t,\alpha} \right)} = {\frac{{{\underset{\_}{d}}_{g}^{+}(m)}{{\hat{\Gamma}}^{- 1}\left( {m,t,\alpha} \right)}}{{{\underset{\_}{d}}_{g}^{+}(m)}{{\hat{\Gamma}}^{- 1}\left( {m,t,\alpha} \right)}{{\underset{\_}{d}}_{g}(m)}} \cdot {\overset{\rightarrow}{V}\left( {m,t,\alpha} \right)}}} \\{{Right}\quad {channel}\text{:}} & {{V_{right}\left( {m,t,\alpha} \right)} = {\frac{{{\underset{\_}{d}}_{d}^{+}(m)}{{\hat{\Gamma}}^{- 1}\left( {m,t,\alpha} \right)}}{{{\underset{\_}{d}}_{d}^{+}(m)}{{\hat{\Gamma}}^{- 1}\left( {m,t,\alpha} \right)}{{\underset{\_}{d}}_{d}(m)}} \cdot {\overset{\rightarrow}{V}\left( {m,t,\alpha} \right)}}}\end{matrix}$

[0046] For a given setpoint direction, the treatment builds aconventional channel and a noise reference having a zero in the setpointdirection, then it subtracts coherently the reference signal from thesignal of the conventional channel.

[0047] Consequently:

[0048] in the target direction: this is excluded from the noisereference, and the treatment achieves optimal gain relative to theambient noise;

[0049] in the ambiguous direction, the target is taken into account inthe noise reference and is subtracted as interference, thereby allowinga right/left rejection. This is possible because the echo coming fromthe target has a sufficient level thanks to the coherent treatmentcarried out before the formation of the channels V_(left), V_(right),and also because the duration of estimation of the matrix is shorterthan the duration of the echo.

1. A signal processing method for a towed linear antenna, notably toresolve right-left ambiguity, said antenna comprising a set ofhydrophone multiplets each with n≧3 hydrophones spread over a straightsection of the longitudinal axis of said antenna, comprising the stepsof: measuring the roll angle of each multiplet relative to the verticalwhen the signals of said hydrophones and said roll angles are used top≧3 linear sub-antennas, forming M azimuth channels with eachsub-antenna; and 2 adaptive right-left channels are formed from said psub-antennas for each direction corresponding to each azimuth channel.2. The process according to claim 1, wherein a coherent treatment usedto form said M azimuth channels.
 3. The process according to claim 1,wherein to form said right-left channels the inverse interspectralmatrix fry {acute over (Γ)}⁻¹ is estimated on N+1 time samples and A+1Doppler channels, then two dephasing vectors d_(r) and d_(l) associatedwith the right and left channels are determined for each azimuth channeland the channels V_(right) and V_(left) are determined from thesevectors and this inverse matrix.
 4. The process according to claim 2,wherein to form said right-left channels the inverse interspectralmatrix {acute over (Γ)}⁻¹ is estimated on N+1 time samples and A+1Doppler channels, then two dephasing vectors dr and d associated withthe right and left channels are determined (403) for each azimuthchannel and the channels V_(right) and V_(left) are determined fromthese vectors and this inverse matrix.
 5. The process according to claim1, wherein inclinometers are used to determine said roll angles.
 6. Theprocess according to claim 2, wherein inclinometers are used todetermine said roll angles.
 7. The process according to claim 3, whereininclinometers are used to determine said roll angles.
 8. The processaccording to claim 4, wherein inclinometers are used to determine saidroll angles.
 9. The process according to claim 1, wherein n=p=3.
 10. Theprocess according to claim 2, wherein n=p=3.
 11. The process accordingto claim 3, wherein n=p=3.
 12. The process according to claim 4, whereinn=p=3.
 13. The-process according to claim 5, wherein n=p=3.
 14. Theprocess according to claim 6, wherein n=p=3.
 15. The process accordingto claim 7, wherein n=p=3.
 16. The process according to claim 8, whereinn=p=3.